1. Field of the Invention
The present invention relates to a digital hierarchy transmission system, and more particularly to a reframer and a Loss Of Frame(LOF) check apparatus.
2. Background of the Related Art
There are several digital hierarchy standards, such as, North American, European, and Japanese hierarchies, in the current communication network. A digital hierarchy includes Digital Signal(DS)-1˜DS-4 hierarchy signals of the North American standard and of the European standard defined by the International Telecommunication Union-Telecommunication Sector ITU-T and the Bell system. Two digital hierarchies are called Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH).
The North American type PDH includes DS-1(1.544 Mbps), DS-1C(3.152 Mbps), DS-2(6.312 Mbps), DS-3(44.736 Mbps), DS-4(274.176 Mbps), etc. The European type PDH includes DS-1E (2.048 Mbps), DS-2E (8.448 Mbps), DS-3E (34.368 Mbps), DS-4E (139.264 Mbps), DS-5E (564.992 Mbps), etc.
The SDH is a Network Node Interface (NNI) standard which is capable of establishing an international communication network by connecting North American type hierarchies and European type hierarchies together. The SDH may be used in the Synchronous Optical Network (SONET) connecting standard for the North American hierarchy, but a basic transmission rate or a frame type is not similar between the hierarchies.
A SDH transmission system provides multiplexing/demultiplexing for mapping digital hierarchy signals, such as DS-1˜DS-4 and DS1E˜DS4E to/from a STM-n signal. The transmission system transmits information by frames having a constant period.
Transmission apparatuses using the SDH are very efficient in network management and providing convenient functions for supervising transmission failures such as bit error failures. A standardization to one communication protocol level in the SDH allows for compatibility with other providers without losing efficiency.
As shown in FIG. 1A, a DS-4E frame in a PDH structure has 488*6 bits (or 61*6 bytes) for a total of 2928 bits (or 366 bytes), according to ITU-T recommendations. The DS-4E (E4) frame includes a 12 bit-Frame Alignment Signal (FAS) for identifying a frame start point, and a Remote Defect Indication (RDI) bit following the FAS. An alarm state is identified when the 12 bit FAS is “111110100000” and the following RDI bit is “1”. The DS-4E frame also includes position fitting bits C and S, a national identification Z, and tributary information bits.
A sending party inserts a FAS to each frame and sends the frames to a receiving party, that detects the FAS and identifies a frame start point. The receiving party also processes data signals by checking a pulse sequence received at every frame.
However, the data signal, which is incoming to the receiving party during a power on/off or a reset, may have an improper frame start point. Data errors can readily occur over many frames if improper start points are generated during power on/off or reset. In order to prevent the processing of improper data, an apparatus is needed for improving stabilization of the overall system and for processing normal data by detecting data errors in the frame.